That feature is not built in, but you can modify the appropriate template file (single.php, page.php, etc.) to selectively call wp_login_form(). Here's how:

If you are using the Hidden Content Teaser:

if ( is_teaser_rs() ) {
  $login_args = array(); // see WP doc for available parameters
  wp_login_form( $login_args );
}

If you are not using the Hidden Content Teaser, there is currently no property or template function to distinguish between an actual "not found" status and an RS-imposed 404. But you can do that with something like this in 404.php:

global $wp_query;
$args = $wp_query->query_vars;
$args['suppress_filters'] = true;

if ( $post_check = query_posts( $args ) ) {
   $login_args = array(); // see WP doc for available parameters
   wp_login_form( $login_args );
}